This invention relates generally to field emission displays, and more particularly to field emission displays having transparent cathodes.
A type of cold cathode for field emission displays is described by C. A. Spindt, et. al. xe2x80x9cPhysical properties of thin film field emission cathode with molybdenum conesxe2x80x9d in the Journal of Applied Physics, V47, No. 12. The described devices produce electrons by quantum mechanical tunneling from the emitter surface into a vacuum under the influence of a large electric field. The electron current generated by any emitter is described by the xe2x80x9cFowler-Nordheimxe2x80x9d equation and is influenced by a number of factors including the work-function of the emitting surface and the physical shape and geometry of the emitter cone. These factors are caused by processing conditions and may result in a large degree of variability in emission current within a grouping of emitters.
Flat-panel, field-emission displays are typically addressed by electrical means. Referring to FIG. 1, a typical prior art display is illustrated with a voltage signal applied between a first conductive electrode 11 disposed on a non-conductive substrate 10 and a second conductive electrode 12 overlying the first conductive electrode. The first and second electrodes are electrically isolated from each other by means of electrically non-conductive dielectric layer 13 lying between the electrode layers. Emitter elements 14 disposed between the first and second electrodes are activated by differential signals applied between the electrodes 11 and 12 to cause electrons 15 to be emitted into a vacuum 20. The electrons are drawn to a light-emitting element 31 by the application of a voltage to an anode element 32.
Light emitting elements 31, opposite the electron-emitting elements 14, generate light responsive to bombardment by electrons emitted from the corresponding emitting element 14. Typically, a large number of electron-emitting elements are associated with each light-emitting element, which defines a pixel. The light-emitting elements 31 are carried by transparent conductive anode viewing screen 32 located parallel to, and spaced from, the electron-emitting elements 14. The anode viewing screen is typically formed on a transparent insulating substrate 30. The light-emitting elements are typically viewed through substrate 30 and transparent anode 32.
Since there can be a large variance in the emission current from emitter to emitter in any grouping or array of emitters, there can be a corresponding variation in the intensity of light emitted by the light-emitting elements. This variation in intensity causes degradation in picture quality.
In addition, current emission heats the emitter. The emission current increases as the temperature of the emitter increases causing a positive feedback condition which can result in the thermal destruction of the emitter and/or arcing of the emitter to the anode causing destruction of the display.
In order to limit current in any emitter and to normalize emission current within the grouping of emitters, Kane, (U.S. Pat. No. 5,142,184) describes the imposition of a series ballast resistor 16 between the first conductive electrode 11 and emitter element 14. The teachings of Kane, and other prior art, describe various configurations of resistive elements interconnecting, and disposed between, the first conductive electrode and the electron-emitting element. Typical prior art field emission displays, FIG. 2, comprise a plurality of substantially parallel first conductive electrodes 11 disposed in a generally horizontal direction on a non-conductive substrate, and a like plurality of second conductive electrodes 12 disposed in a direction generally perpendicular to, and overlying, the first conductive electrodes. The first and second electrode arrays are electrically isolated from each other by means of the electrically non-conductive dielectric layer 13 lying between the layers containing the electrode arrays. Electron-emitting elements 14 are disposed along each first conductive electrode at the intersection of the first and second conductive electrodes. When electrical signals are applied between first and second conductive electrodes, electrons are emitted from the electron-emitting elements located at the intersection of the first and second electrodes. Each electrode is made to be as narrow in width as possible to provide a large number of electrodes per display width. The resistance 17 of the electrode must be small in order to provide electrical addressing signals from one end of the electrode to the other without attenuation and consequent loss of signal fidelity. Consequently, electrodes are typically constructed from opaque metallic materials such as aluminum. Typical resistivities of these materials may be in the range of 2-5 micro-ohm-cm.
Referring to FIGS. 1 and 3, the ballast resistors 16 in series with the electron-emitting element and the first conductive electrode must be of a high resistance to provide sufficient retardation of excess current in the emitter element. This resistance is typically 105 to 107 Ohms. Ballast resistors 16 are electrically in parallel with the intrinsic series resistance 17 of the first conductive electrode 11. Ballast resistors must be significantly higher in resistance than the electrode resistance along the electrode from one grouping of emitters to any other grouping of emitters. This favorable ratio minimizes nonuniformity of electron emission as a function of distance along the electrode. This is shown schematically in FIG. 3 to provide illustration. A disadvantage of this type of ballasting is that the resistance of resistor 16 is dependent on geometry. FIG. 10 shows various construction errors encountered in the photoresist and etching processes used to define the placement and shape of elements 11, 14 and 16. FIGS. 10a, 10b and 10c show varying lengths of resistor 16 due to x and y deviations in photoresist mask alignment causing varying resistance from one display to another. FIG. 10d shows an alignment error through rotation of the cones 14 to the resistors 16 and metal line 11. This causes a high variation in ballast resistance and non-uniform emission. FIG. 10e show an abnormally thin ballast resistor 16 at emitter 14a due to undercutting of photoresist, and FIG. 10f shows a necking down of ballast resistor 16 at emitter 14a that can occur where a right angle is formed between two elements such as elements 11 and 16.
A second prior art display is described in U.S. Pat. No. 5,646,479, a portion of which is shown in the cross-sectional view, FIG. 4. This display is viewed so that the light emitting element 31 may be viewed through substrate 40. This configuration has several advantages, including the possibility of a reflective electrode 52. This is a technique well known in the art for increasing the amount of light emitted as a function of the electron beam current. This art describes a display comprising a transparent first electrode 41 on which is disposed and mounted a plurality of emitter cones 14. A second transparent, conductive electrode 42 is located plane-parallel to the first conductive transparent electrode and separated by a layer of dielectric material 13. An electrical signal applied between the first and second transparent conductive electrode causes electrons 15 to be emitted from the emitter cones 14 disposed on the first transparent conductive electrode. The resistivity of the transparent conductive electrode 41 is selected so as to provide electrical short protection in the event of such a failure at any emitter. However, as may be seen from the illustrative schematic representation, FIG. 5, the resistance 17 between the signal source and emitter cone 14 is a variable function of the distance between the cone and the location of the applied electrical signal 5. This variability in resistance can result in attenuation of electrical signal in the regions of highest resistance and increased emission current in the regions closest to the source of electrical signal application. This non-uniformity of electron emission can cause consequent loss of image quality when this structure is used as cathode in a display. Additionally, in order to achieve high light transmission through the transparent, conductive electrode, the conductive film must be thin. This increases the overall sheet resistance of the conductive film, limiting the maximum size of this type of display.
Accordingly, it is an object of the invention to provide a field emission display with uniformity of electron emission spatially distributed throughout the cathode.
It is another object to provide improved method for fabricating said cathode.
It is another object to provide a method for producing series emitter ballasting resistors that depend primarily on the thickness of the ballast material and not on the width and length.
It is another object to provide improved protection against electrical shorting between transparent conductive electrodes.
It is another object to provide for increased light transmission through said field emission cathode.
It is another object to reduce the resistance between the source of electrical signal and the location of the emitter cone.
There is provided a field emission display in which the cathode includes a ballast resistor between the electron-emitting cone and the first transparent conductive electrode which connects the cones to the source of the electrical signal, and in which the distributed resistance of the transparent conductive electrode is minimized by creation of a low-resistance region lying outside the region of electron-emitting cones.